546 THE SPINAL CORD. 



short distance downward), but eventually either go, as we shall see, to form 

 the median posterior tract or make their way back into the gray matter at 

 the base of the posterior horn and thus join the vesicular cylinder, though 

 some are said to be continued on through the gray matter into the anterior 

 horn. The other smaller bundle placed to the outside of the former, and 

 called the lateral bundle (Fig. 121, Pr), may be again divided into an inter- 

 mediate bundle (Fig. 122, Pr) lying next to the median bundle, and into a 

 still more lateral bundle (Fig. 122, Pr"). The former, consisting also of 

 coarse fibres, plunges directly through the substance of Rolando at the ex- 

 tremity of, and so into the gray matter of the horn, where the fibres chang- 

 ing their direction run in part at least longitudinally in the gray matter in 

 bundles known as " the longitudinal bundles of the posterior horn " (Figs. 

 121, 122, r.f. p.), some of which appear to pass on to the anterior horn. 

 The small, most external or lateral portion of the lateral bundle, consisting of 

 fine fibres and sometimes spoken of as the lateral bundle, on entering the cord 

 at once ascends for some distance, and thus forms the thin layer of fine fibres, 

 the posterior marginal zone or Lissauer's zone, indicated in Fig. 122 by m. t., 

 which lies between the actual extremity of the horn and the surface of the 

 cord, and in the upper regions of the cord (c/. Fig. 121, p') runs some way 

 upward on the lateral margin of the horn between the gray matter and the 

 crossed pyramidal tract. As it ascends this layer continually gives off fibres 

 to the gray matter of the posterior horn in the cells of which they appear 

 to end. 



Thus, while part of the median bundle does not join the gray matter at 

 all but goes to form the median posterior tract, the rest of that bundle and 

 all the other fibres of the root, sooner or later, join the gray matter either 

 of the posterior horn or of some other part. 



483. The special features of the several regions of the spinal cord. The 

 cord begins below in the slender filament called ihefilum terminate, which 

 lying in the vertebral canal, in the midst of the mass of nerve-roots called 

 the cauda equina, rapidly enlarges at about the level of the first lumbar ver- 

 tebra into the conus medullaris. This may be regarded as the beginning of 

 the lower portion of a fusiform enlargement of the cord known as the lumbar 

 swelling, which reaches as high as about the attachment of the roots of the 

 twelfth or eleventh thoracic nerve at the level of the eighth thoracic ver- 

 tebra, the broadest part of the swelling being about opposite the third lumbar 

 nerve. Above the lumbar swelling, through the thoracic region the some- 

 what narrowed cord retains about the same diameter until it reaches the 

 level of the first or second thoracic nerve opposite the seventh cervical ver- 

 tebra where a second fusiform enlargement, the cervical swelling ', broader and 

 longer than the lumbar swelling, begins. The broadest part of the cervical 

 swelling is about opposite to the fifth or sixth cervical nerve ; from thence 

 the diameter of the cord becomes gradually somewhat less until it begins to 

 expand into the bulb, but even in the highest part is greater than in the 

 thoracic region. The sectional area of the cord increases therefore from 

 below upward, but not regularly, the irregularity being due to the lumbar 

 and cervical swellings. The extremity of the filum terminale is said to con- 

 sist entirely of neuroglia closely invested by the membranes, even the central 

 canal being absent. A little higher up the central canal begins, and nerve- 

 cells with nerve-fibres make their appearance in the neuroglia ; thus a kind 

 of gray matter covered by a thin superficial layer of white matter is estab- 

 lished. We have already referred to the peculiar features of the lower end 

 of the conus ( 477) ; but higher up the canal becomes central and small, 

 the posterior columns are developed, and the gray matter contains more 

 nervous elements and relatively less neuroglia, becomes in fact ordinary gray 



